What makes you think the Lindens are going to open source the server code, which will basically allow anyone to make their own sims independent of the Second Life grid, when the tier for owning land or an island is the main source of income for Linden Lab?
Honestly, explain this to me. What am I missing here? You could make the case that while open sourced, they will sell it to the potential coder. But then, the customer will take that and reverse engineer that and likely issue it around the internet where you won't be able to place that genie back into the bottle. You wouldn't catch these people easily either if they run their sims offline or on secure networks. Even a legit buyer gets the code and then pays no further costs to Linden Lab. It's a temporary source of income which will end when those who will buy the server code buy it and everyone else sticks around. This would lead to tier income dropping for the Lindens.
By my reckoning, they lose in any case. I suppose they could always offer a fee for hooking into the main grid, but why bother if you can start your own Second Life grid, advertise it on some freebie website host and attract people onto your private grid. More people would be drained off as they leave to try out these other grids.
I just can't see how open source would help the Lindens in the long run. In the short run they benefit from an army of coders picking the code apart and fixing it (much like what was done with the SL client and the Nicholaz client), but the point isn't setting up better servers but that separate grids would pop up and could even usurp the main grid. That is what would nail the Lindens long run. They would need something along the lines of protectionism enforced by the recording industry.
And this is why I believe the Lindens will never open source the servers completely if at all. It's a pipe dream kicked around every so often. It will never really be released unless Linden Lab itself was about to fold or if their business model changes completely (some model not based upon tier and land sells). Until then, I highly doubt we'll see open sourced servers for some time.